The present invention relates generally to object-based high level programming environments, and more particularly, to techniques suitable for customization of a Java runtime environment.
One of the goals of high level languages is to provide a portable programming environment such that the computer programs may easily be ported to another computer platform. High level languages such as “C” provide a level of abstraction from the underlying computer architecture and their success is well evidenced from the fact that most computer applications are now written in a high level language.
Portability has been taken to new heights with the advent of the World Wide Web (“the Web”) which is an interface protocol for the Internet which allows communication between diverse computer platforms through a graphical interface. Computers communicating over the Web are able to download and execute small applications called applets. Given that applets may be executed on a diverse assortment of computer platforms, the applets are typically executed by a Java™ virtual machine.
Recently, the Java programming environment has become quite popular. The Java programming language is a language that is designed to be portable enough to be executed on a wide range of computers ranging from small devices (e.g., pagers, cell phones and smart cards) up to supercomputers. Computer programs written in the Java programming language (and other languages) may be compiled into Java Bytecode instructions that are suitable for execution by a Java virtual machine implementation. The Java virtual machine is commonly implemented in software by means of an interpreter for the Java virtual machine instruction set but, in general, may be software, hardware, or both. A particular Java virtual machine implementation and corresponding support libraries together constitute a Java runtime environment.
Computer programs in the Java programming language are arranged in one or more classes or interfaces (referred to herein jointly as classes or class files). Such programs are generally platform, i.e., hardware and operating system, independent. As such, these computer programs may be executed without modification on any computer that is able to run an implementation of the Java runtime environment.
Object-oriented classes written in the Java programming language are compiled to a particular binary format called the “class file format.” The class file includes various components associated with a single class. These components can be, for example, methods and/or interfaces associated with the class. In addition, the class file format can include a significant amount of ancillary information that is associated with the class. The class file format (as well as the general operation of the Java virtual machine) is described in some detail in The Java Virtual Machine Specification, Second Edition, by Tim Lindholm and Frank Yellin, which is hereby incorporated herein by reference.
FIG. 1A shows a progression of a simple piece of a Java source code 101 through execution by an interpreter, the Java virtual machine. The Java source code 101 includes the classic Hello World program written in Java. The source code is then input into a Bytecode compiler 103 that compiles the source code into Bytecodes. The Bytecodes are virtual machine instructions as they will be executed by a software emulated computer. Typically, virtual machine instructions are generic (i.e., not designed for any specific microprocessor or computer architecture) but this is not required. The Bytecode compiler outputs a Java class file 105 that includes the Bytecodes for the Java program. The Java class file is input into a Java virtual machine 107. The Java virtual machine is an interpreter that decodes and executes the Bytecodes in the Java class file. The Java virtual machine is an interpreter, but is commonly referred to as a virtual machine as it emulates a microprocessor or computer architecture in software (e.g., the microprocessor or computer architecture may not exist in hardware).
FIG. 1B illustrates a simplified class file 100. As shown in FIG. 1B, the class file 100 includes a constant pool 102 portion, interfaces portion 104, fields portion 106, methods portion 108, and attributes portion 110. The attributes (or attributes table) 110 portion represents the attributes associated with the class file 100. This allows for one or more attributes to be defined, each of which can be associated with one or more components of the class file. As is known to those skilled in the art, the Java virtual machine implementations are allowed to define and use various attributes. In addition, the virtual machine's implementations ignore attributes that they do not recognize. Thus, a class file may contain one or more attributes, all or none of which may be recognized by a particular virtual machine implementation.
As is known to those skilled in the art, execution of a Java application typically requires various Java features to be available at runtime. Unfortunately, however, one problem with conventional virtual machine implementation is that typically most of the available features are loaded regardless of whether they are going to be used at runtime by a particular Java application. This, of course, can result in a grossly inefficient use of system resources. In some circumstances, particularly in systems with limited computing power and/or memory, this inefficient use of resources is a serious disadvantage. As such, it is highly desirable to customize Java runtime environments so as to optimize performance of Java applications.
Accordingly, there is a need for techniques that allow customization of Java runtime environment of virtual machines that operate with limited computing power and/or memory (e.g., embedded systems).